When you think of menacing, grizzled, veteran Watchmen patrolling the dirty streets of Marienburg, what is the first image that comes to you mind? If you said large, flopping black hats, then you would be absolutely correct.
The city of canals and criminals is patrolled (at least on land) by an organisation known as the Honourable Company of Lamplighters and Watchmen, or Black Caps, named after the hats they wear. No one can really remember if this slang term was first used as an insult, but it is now used by everyone - even officials within the organisation.
Depending on where you are in the city, you might see a Watch Post every few streets, or at the corner of every main intersection. These posts hold anywhere from 4 to 12 Black Caps (depending on time of day and roughness of area), and they regularly patrol their given beats and report to their Barracks. Each district in the city has its own Barracks, which can hold anywhere up to 100 Black Caps, depending on the temperament of the area, which itself is overseen by the Ward House, a larger office (come fortress) that is located as the seat of power for the Company in each Ward. Overlooking all of this, however, is the Headquarters located in the Palace District, right near the High Court.
Despite this rigid and regulated exterior, the individual Wards, and Barracks in some cases, are largely self sufficient - their members going anywhere from mere Watchmen, to Guards, to active Crime Fighters, to "official" vigilantes. It really depends on what the attitude of the Ward Captain is in the regard.
For some Ward Captains, like Theophilius Graveland of Suiddock, the Black Caps are whatever they have to be to keep the peace. This means that at any one time they can be thugs roughing up young Agitators, or paragons of virtue as they bust in the heads of some criminal filth. They've even been known to have back-room dealings with the Guild We've Never Heard Of in the hopes that an agreement is better than blind fighting.
So remember, when running around the streets of Marienburg, if you're ever in trouble, you can always turn to the Black Caps for help. That is, if you have a few spare coins you're willing to part with. Or supposing that it isn't the Black Caps themselves who are chasing you...